Not Applicable
Not Applicable
1. Technical Field
This invention relates in general to communications and, more particularly, to automatic frequency control systems.
2. Description of the Related Art
Automatic frequency control (AFC) systems are important parts of radio communication systems, such as the wide-band CDMA (Code Division Multiple Access) systems. An AFC system reduces small inconsistencies between the carrier frequency of a transmitter and the carrier frequency of a receiver. The inconsistency is referred to as xe2x80x9cfrequency offset.xe2x80x9d If the carrier frequency offset is large, demodulated signals at a receiver will include frequency rotation, which leads to the increase of the bit error rate.
The carrier frequency offset can be improved through the use of an AFC system. The typical AFC system performs two steps: (1) it estimates the carrier frequency offset from the demodulated signals, and (2) based on the estimated frequency offset, a voltage controlled oscillator (VCO) is controlled so as to reduce the carrier frequency offset.
Many research interests have been concerned with the first step, and many efforts have improved the accuracy of the carrier frequency estimation and convergence speed of the AFC. These improvements are discussed in: Urs Fawer, a Coherent Spread-Spectrum Diversity-Receiver with AFC for Multipath Fading Channels, IEEE Transactions on Communications, Vol. 42, No. 2/3/4, pp 1300-1311 (February 1994) and Kazuo Tanada, Fumio Ishizu, Keishi Murahami and Tadashi Fujino, performance of AFC Scheme Based on Half-Symbol Differential Detection,xe2x80x9d IEICE Tech.Rep.RSC96-25, pp 1-6 (June 1996). However, as a final step of frequency offset estimation, these methods include an arctangent calculation. Using arctangent results in the increase of the computational complexity of digital signal processing systems. On the other hand, Hideki Ishizuki and Shinichi Sato, A Study on Automatic Frequency Control for Spread Spectrum Mobile Radio Communication, IEICE Tech.Rep.RSC96-105, pp.45-50 (November 1996), presents a method which approximately estimates the frequency offset and proposes to apply random walk to improve the convergence of the AFC. This method uses the Doppler frequency to stabilize the system, which is also difficult to implement.
Therefore, a need has arisen for an AFC system with a simple calculation for offset frequency.
In the present invention, frequency of an oscillator is controlled by determining real and imaginary components of an angle of rotation between a carrier frequency and output of the controllable oscillator and generating a sinusoidal approximation of the angle of rotation from said real and imaginary components and an offset to the sinusoidal approximation responsive to the sign of the sinusoidal approximation to produce an enhanced approximation. The oscillator is adjusted responsive to the enhanced approximation.
The present invention provides significant advantages over the prior art. The approximation of the angle of rotation makes it possible to extend the pull-in range of the AFC greater than that of conventional approximation techniques and improves the convergence speed of the AFC system when initial carrier frequency offset is large, while maintaining a simple approximation approach.